In the Advanced Liquid Metal Reactor (ALMR), a reactor core of fissionable fuel is submerged in a hot liquid metal, such as liquid sodium, within a reactor vessel. The liquid metal is used for cooling the reactor core, with the heat absorbed thereby being used to produce power in a conventional manner.
A known version of an ALMR plant has a concrete silo which is annular or circular. The silo is preferably disposed underground and contains concentrically therein an annular containment vessel in which is concentrically disposed a reactor vessel having a nuclear reactor core submerged in a liquid metal coolant such as liquid sodium. The annular space between the reactor and containment vessels is filled with an inert gas such as argon. The reactor and containment vessels are supported or suspended vertically downward from an upper frame, which in turn is supported on the concrete silo by a plurality of conventional seismic isolators to maintain the structural integrity of the containment and reactor vessels during earthquakes and allow uncoupled movement between those vessels and the surrounding silo.
Operation of the reactor is controlled by neutron-absorbing control rods which are selectively inserted into or withdrawn from the reactor core. During operation of the reactor, it may be necessary to shut down the fission reaction of the fuel for the purpose of responding to an emergency condition or performing routine maintenance. The reactor is shut down by inserting the control rods into the core of fissionable fuel to deprive the fuel of the needed fission-producing neutrons. However, residual decay heat continues to be generated from the core for a certain time. This heat must be dissipated from the shut-down reactor.
The heat capacity of the liquid metal coolant and adjacent reactor structure aid in dissipating the residual heat. For instance, heat is transferred by thermal radiation from the reactor vessel to the containment vessel. As a result, the containment vessel experiences an increase in temperature. Heat from the containment vessel will also radiate outwardly toward a concrete silo spaced outwardly therefrom. These structures may not be able to withstand prolonged high temperatures. For example, the concrete making up the walls of the typical silo may splay and crack when subjected to high temperatures.
To prevent excessive heating of these components, a system for heat removal is provided. One of the heat removal systems incorporated in the ALMR is entirely passive and operates continuously by the inherent processes of conduction and radiation of heat and natural convection of fluids. This safety-related system, referred to as the reactor vessel auxiliary cooling system (RVACS), is shown schematically in FIG. 1. Heat is transported from the reactor core to the reactor vessel 15 by natural convection of liquid sodium. The heat is then conducted through the reactor vessel wall. Heat transfer from the reactor vessel outside surface to the colder containment vessel 7 across a gap space 16 filled with an inert gas, such as argon, is almost entirely by thermal radiation. A heat collector cylinder 4 is disposed concentrically between the containment vessel 7 and the silo 5 to define a hot air riser 6 between the containment vessel and the inner surface of the heat collector cylinder, and a cold air downcomer 3 between the silo and the outer surface of the heat collector cylinder. Heat is transferred from the containment vessel 7 to the air in the hot air riser 6. The inner surface of heat collector cylinder 4 receives thermal radiation from the containment vessel, with the heat therefrom being transferred by natural convection into the rising air for upward flow to remove the heat via air outlets 9. Heat transfer from the containment vessel outer surface is approximately 50% by natural convection to the naturally convecting air in the hot air riser 6 and 50% by radiation to the heat collector cylinder 4.
Heating of the air in the riser 6 by the two surrounding hot steel surfaces induces natural air draft in the system, with atmospheric air entering through four air inlets 1 above ground level. The air is ducted to the cold air downcomer 3 via the inlet plenum 2, then to the bottom of the concrete silo 5, where it turns and enters the hot air riser 6. The hot air is ducted to the four air outlets 9 above ground level via the outlet plenum 8. The outer surface of heat collector cylinder 4 is covered with thermal insulation (not shown) to reduce transfer of heat from heat collector cylinder 4 into silo 5 and into the air flowing downward in cold air downcomer 3. The greater the differential in temperature between the relatively cold downcomer air and the relatively hot air within the riser, the greater will be the degree of natural circulation for driving the air cooling passively, e.g., without motor-driven pumps.
The overall heat removal rate of the RVACS increases with temperature and is controlled to a large degree in the air riser gap by convective heat transfer from enclosing surfaces. Thus, if it were possible to increase the convective heat transfer on these surfaces or increase the exposed surface area, a larger decay heat load would be rejected by the RVACS at any given reactor assembly temperature.
Two methods of enhancing the RVACS performance by such means are respectively described in U.S. Pat. No. 5,043,135 to Hunsbedt et al., entitled "Method for Passive Cooling Liquid Metal Cooled Nuclear Reactors and Systems Thereof", and in U.S. Pat. No. 5,339,340 to Hunsbedt, entitled "Method for Enhancing Air-Side Heat Transfer to Achieve Improved Reactor Air-Cooling System Performance".
U.S. Pat. No. 5,043,135 describes an air-side heat transfer surface preparation technique that results in a higher air-side convective heat transfer rate. It involves the creation of surface roughness by placement of protrusions 10 (see FIG. 2) that disturb the thermal boundary layer near the hot steel walls.
An additional enhancement method described in U.S. Pat. No. 5,339,340 utilizes the air-side enhancement method of U.S. Pat. No. 5,043,135 in combination with an additional, perforated collector cylinder 11 (see FIG. 2) placed in the air stream. The use of a perforated steel cylinder is unique in that the degree and shape of the perforations can be adjusted and selected such that optimum air-side heat transfer is achieved.
The supplementary decay heat removal system which is the subject of the present invention can be used by itself but is more effective when used in combination with the enhancements of U.S. Pat. Nos. 5,043,135 and 5,339,340. This approach is assumed in the following discussion.